Reviving a WW2 Dynamotor, How Tube Radios Went Mobile During the War

this instructable requires that you have above average mechanical dexterity and have worked with high voltages. if you have done things such as successfully repack bearings on a bicycle hub for example, you have the mechanical part down. if you have successfully wired electrical outlets, you should be good using common sense on the electrical portion. this instructable is geared towards small dyno's used in old military radio receivers but the info presented can be applied to larger units of similar design.

what is a dynamotor?

a dynamotor or motor-generator is one way low voltage DC was converted into high voltage DC to run tube radios. this type of device was commonly used as a power source for mobile commercial and military radios from the days of the first mobile radios up until the late 1950's when transistors began to be used in inverter circuits to create the high voltages needed for tubes.

dynamotors are still fairly common finds at radio fleamarkets. they almost always need a little refreshing to get them going. resist the urge to just power it up. hardened grease is hell on bearings and these machines typically operate in the 5k-7k rpm range. hardened grease can also lock the armature up making your power supply very angry as it will see a dead short.

unless tampered with, all military dynamotors will have ID tags. this tag has a wealth of info about your unit. not all dyno's will be military. a great many of the ones out there are thanks to the vast amounts of ww2 and korean war era surplus that was sold to the public 50+ years ago.

a military unit will likely have a "BC" or "DM" number. the BC number will tell you what radio it went into, the DM number is the dynamotor type. sometimes the same dyno could be used on different radios. the military units will also have full specs on duty cycle, supply voltage, draw, and what's it rated output is. output is usually a rather high voltage at a low mA rating.

dyno's used on receivers are typically the size of a coke can or a little bigger. dyno's used on transmitters can be huge and heavy. we're talking a cylinder shaped object weighing 30+ pounds. the unit we are working on here is for a ww2 receiver.

aside from military units, the second most common dyno's found will be from early motorola and GE commercial mobile radios. these old radios are also well documented. it will be worth googling any numbers you find on the unit to see what it came from. since these were typically 2 way radios, the dyno will be on the large side as it had to support a transmitter.

in order to get in, you'll have to pop off the end covers. these covers will either have bolts on the ends or may have a side bolts that you loosen and give the cover a slight turn to release it. these covers can often be removed without the need to remove the dyno from its mounting plate.

these covers can be tough to remove. you may have no choice but to mark up the paint a little bit by using a screwdriver at the seam of the cover and motor body. some light tapping will help get the cover to start moving. work your way around to get the cover off.

in some cases the cover may have lubricating instructions inside it like the one pictured. 

let's move on to tear down.

you should now be looking at what looks like an electric motor with two commutators. one end of the motor is the low voltage input and the other is the high voltage output. what you have is a generator and a motor sharing the same housing and armature. as the motor turns, the generator which is wound on the same shaft goes along for the ride.

on each end of the dyno, there will be a removable endcap about the size of a quarter. this cap retains the grease for the bearings on the ends of the armature shaft. the cap will typically be held on by 2 or 4 screws. remove a cap. under the cap there may be a seal and maybe some washers. those washers are spacers to control end play on the armature shaft. make sure to keep those in order so you reinstall them properly. you might have to use a thin screwdriver to pop up the seal if present and the washers.

if after you remove the screws the cap wont pop off. use the handle end of a large screwdriver and give the cap a brisk tap on its edge. that should knock the cap off for you.

let's see what the grease looks like.

if the grease in there has the consistency of jelly and there's no rust, you're looking good. all you'll have to do is top off the grease in there and put the cap back on. once that's done you clean the commutator and check the brushes and you're ready to power up.

if the grease in there has the consistency of peanut butter or nutella, further tear down is recommended. the stuff on my finger in the pic was like nutella and required that this unit be taken apart.

does your dyno have a shaft mounted fan? if so, you need to remove the fan first. this is MUCH easier if you have access to an air driven impact wrench. if you do not have access to an air impact, an electric impact *might* work but you really need the speed of an air tool. no impact tools? read on.

take a marker or preferably an awl and put a small line on each armature end cap that goes across both the end cap and the surface it mates with. this will serve as a guide for you to line things up properly later on reassembly.

pick an end to remove. if your dyno has a fan, pick the end with no fan. remove the brush holders and brushes. these are the round bakelite caps that are on each end of the dyno. one is fuzzily pictured in pic 1. usually there is 4 but some bigger dynos may have 6. keep track of where these go in your dyno and which side faces up. over time they have worn to fit the curvature of the armature properly. you don't want to mix them up. do NOT get any oils on these. its detrimental to them.

next remove the screws holding the terminals that connect to the brush holders. there is one pictured in pic 1, just left of the black round brush cap. in this case its the round flathead fastener. however many brush caps you have is how many of these leads you need to remove to remove the armature end cap. move the leads aside so the cap will come off without damaging the wiring.

remove the fasteners that hold the armature end cap onto the body of the motor. there may be 2-4 of these. see pic 5 & 6. once those are out, take the plastic end of a large screwdriver and give the endcap a few brisk taps. you're looking to cause a small separation between the endcap and the main motor body. once you've done that, take a flathead and work your way around the motor body slowly prying the cap off. see pic 7.

if all goes well, you will end up with either just an endcap in your hand or an armature and an endcap as in pic 8. if your unit had a fan and you had no impact tool, now comes fan removal time. take a THICK piece of fabric like a shamwow, or a piece of leather and wrap it around the commutator a couple of times. use a pair of slip joint pliers to grab onto the commutator. the leather or fabric should protect the commutator from damage. do NOT do this without protecting the commutator. with the commutator clamped down you should be able to remove the fastener holding the fan on. use some wd40 on the nut holding the fan to make your life easier. this is a risky procedure.

your endcap came off with the armature attached? have no fear. support endcap as shown in the pic. you are supporting the endcap on the jaws of a vise while the armature hangs down freely. find a nut driver or socket that fits on the outer bearing race. the idea is to use something to tap the bearing and thus the armature, out of the endcap. this may require 3 hands so find a buddy. the armature can fall out at any moment and you do not want it impacting the floor.

you need to get the old grease out of the bearings. these bearings don't always slide off the armature shaft so you might have to work with them in place. here's two ways to get this done...
1) get a can of electrical motor cleaner, go outside, and blast those bearings out with motor cleaner. blast from the commutator out, not the other way. you don't want to blast the crud into the commutator. you must use electric motor cleaner for this, anything else can damage the insulating varnish on the armature.
2) find a small container big enough so you can submerge the bearing in a fluid but not the commutator. fill the container with mineral spirits and dip the bearing. spin it, dip it, spin it, dip it. keep doing this till you get all that old grease out. allow the bearings to air dry.

clean up those commutators!

i have successfully used scotchbrite for this. wrap it around the commutator, grip it with your hand and twist the armature around a few times. that should make a huge difference. wrap your scotchbrite so it doesn't rub against the wiring in the armature, just the commutator. pic 3 shows what a shiny and clean commutator looks like. note how there are small grooves in the commutator? that's why it's very important the old brushes go back in the same exact place they came out of and they face the right way. if the grooving is bad, proper technique is to mount the armature in a lathe, turn it till you've removed the groove, undercut the mica, and use new brushes. that procedure is beyond the scope of this instructable.

once your done with the light cleanup, use a small screwdriver or awl to clean any crud out of the grooves in the commutator. be gentle doing this. the copper used in the commutator is very soft.

time to repack the bearings and get ready for reassembly.

some units want a proper bearing repack, others want you to just slather it on a bit. the reason for this has to do with the type of seal they used. a fully packed bearing can sling grease if it doesn't have any seals holding the grease in. the model i'm working on does not call for a proper bearing repack. they want the slather job. they even tell you on the instructions in the end covers.

slather it is. i dabbed some fresh wheel bearing grease on there and worked it in a little with my finger tips. i spun the bearing and then smeared it in some more. all done. for a proper repack you need a different technique. there's plenty of tutorials on DIY auto repair sites about bearing packing so we wont get into that.

carefully slide armature back into housing WITHOUT touching the commutators. take your scotchbrite and go around the edges of the armature end cap. this will facilitate it's re-installation as you have removed some of the crud that built up over the years. slide the armature end cap back on the main motor housing making sure you have it lined up right. remember those marks you scribed on the armature end caps and the motor housing earlier? this where they help you line things up. insert pass through bolts that hold the end caps in place and tighten those up. put your armature shaft end cap bearing cover back on if you haven't already or you'll sling grease all over the place.

re-attach leads going to brush holders. re-insert brushes and springs being careful to put them in their proper place and direction (you did note where they go right?). your brushes should be at least 1/4" in length on these small motors. if you have less than that left, you need to replace them if you plan on actually using your dyno for more than an occasional display item. new brushes may be obtainable at an electric motor shop. note that the brushes may have a metal retainer that guides the spring. DON'T forget that. its part of what carries the current.

once you have it all back together, it's time to test.

alligator leads are your friend here!

DANGER HIGH VOLTAGE!
you will now be dealing with hundreds of volts. if you are uncomfortable with this, find somebody who knows how to safely work with high voltages.

most dyno's will be labeled in some way either in ink or by stamp in the casting as which side is HV (high voltage) and which is LV (low voltage). you will be connecting a power supply to the low voltage side. if your dyno is not labeled have no fear. you cant hurt it by hooking low voltage to the wrong end.

i like to use a variable power supply for testing these. inrush current can be substantial so you want something that can provide 10A or more. once they spin up, they draw 1-5 amps unloaded depending on their size. the small units typically free spin at under 2A. run leads from your power supply right to the brushes on the + and - on the low voltage side. connect to the same leads you may have unscrewed earlier when you removed the armature end cap. turn up your supply to a few volts lower than the unit specs at and flip the power on. if all is well, it should start spinning.

dyno's will usually run at even 1/3 their rated voltage without a load. if you're working on a 28v dyno and you only have a 12v supply, it should still spin up. it just wont provide the full current and voltage it specs at.

it spun! ok, power off and take your voltmeter leads and put alligator clips on them. clip those leads to the brush leads for the high voltage side. put your meter on the 1Kv DC scale and switch your power supply on. you should see the voltage climb as the motor spins up. let it run for a bit and keep an eye on that voltage. this will allow the brushes to re-seat.

all good? you can load test your dyno using a 25-40watt incandescent house bulb and a socket with leads on it. small dynos really cant provide sufficient current so despite their higher voltage claims, it gets pulled down when loaded heavily. just for test purposes you wont hurt the dyno. they are very resilient to brief overloads. hook light bulb to leads you had going to your meter from the high voltage side and flip on the power. the dyno should spin up and bulb should come on.

be careful doing this with large dynos. they CAN provide the voltage and current to turn a regular house bulb into a flashbulb!

burnt smells? motor spins up and seems to load up? pulls alot of current?
1) you may have a shorted armature or field winding. this is bummer as they aren't worth fixing really unless you wish to take up the art of motor rewinding.
2) the more probable, you have a shorted filter section. many of these dynos came with a built in RF hash (noise) filter and sometimes even some electrolytic filter caps to smooth things out. these can go bad like any other capacitor and short over time. rebuilding this section is beyond the scope of this instructable but its basically just replacing all the caps in that area with the same values.

it's working but you have lots of sparking at the brushes?
you mixed up or inverted some of the brushes and they are now grinding away till they re-seat. let the dyno run for a bit unloaded and the sparking will settle down as the brushes wear in.

once you put the end cap covers back on you won't be able to clip your alligator leads on anymore. if you want to be able to use your dyno with the caps on, you'll have to do some continuity tracing of the wiring from brushes to see where on the unit's connector is your LV and HV connection points.

if all went well, your dyno should spin up with a very distinct whine. it's a sound any old timer that was in the military and worked a radio will instantly recognize. old dynos are cool machines for the vintage electrical gadget geek but they also have collector value. some old military radio buffs seek these out for their restorations. if you get tired of having it around, keep it in mind that your old dyno may have some resale value to military radio collectors, specially now that it's been serviced.

the short video will let you hear the dyno that was used for this instructable come to life.